Health & Pregnancy

Pregnancy Essentials

A User's Guide to Prenatal Tests

A User's Guide to Prenatal Tests

WebMD Archive

Second Trimester Tests

Here are the prenatal tests you may face in the second trimester:

Maternal serum alpha-fetoprotein (MSAFP) and multiple marker
screening: One or the other is offered routinely at 15 to 18 weeks. The
MSAFP test measures the level of alpha-fetoprotein, a protein produced by the
fetus. Abnormal levels indicate the possibility (not existence) of Down
syndrome or a neural-tube defect such as spina bifida, which can then be
confirmed by ultrasound or amniocentesis.

When blood drawn for the MSAFP test is also used to check levels of the
hormones estriol and hCG, it's called a multiple-marker screening. The multiple
marker test significantly boosts the detection rates for Down syndrome. The
test picks up about 80% of neural tube defects and 70% of Down syndrome, but
many women will have a false-positive screening. About 3% to 5% of women who
have the screening test will have an abnormal reading, but only about 10% of
those women will have a child with a genetic problem.

Ultrasounds: A sonogram may be offered between 18 and 20 weeks for a
variety of reasons, including verifying a due date, checking for multiple
fetuses, investigating complications such as placenta previa (a low-lying
placenta) or slow fetal growth, or detecting malformations like cleft palate.
During the procedure, a device is moved across the abdomen that transmits sound
waves to create an image of the uterus and fetus on a computer monitor. New
three-dimensional sonograms provide an even clearer picture of your baby, but
they aren't available everywhere and it's not clear whether they're any better
than two-dimensional pictures in contributing to a healthy pregnancy or
birth.

Glucose screening: Typically done at about 25 to 28 weeks, this is a
routine test for pregnancy-induced diabetes, which can result in overly large
babies, difficult deliveries and health problems for you and your baby. This
test measures your blood-sugar level an hour after you've had a glass of soda.
If the reading is high, which happens about 20% of the time, you'll take a more
sensitive glucose-tolerance test, in which you drink a glucose solution on an
empty stomach and have your blood drawn once every hour for three hours.

Amniocentesis: This optional diagnostic test is usually performed
between 15 and 18 weeks (but can be done earlier) for women who are 35 or
older, have a higher-than-usual risk of genetic disorders, or whose AFP or
multiple-marker screen test results were suspicious. The procedure is done by
inserting a needle through the abdomen into the amniotic sac and withdrawing
fluid that contains fetal cells. Analysis can detect neural tube defects and
genetic disorders. The miscarriage rate varies depending on the experience of
the physician, averaging about 0.2% to 0.5% at 15 weeks and 2.2% at 11 to 14
weeks, but the test can detect 99% of neural-tube defects and almost 100% of
certain genetic abnormalities.